The Collaborative International Dictionary of English v.0.48:

Safety bicycle \Safety bicycle\
   A bicycle with equal or nearly equal wheels, usually about 28
   inches diameter, driven by pedals connected to the rear
   (driving) wheel by a multiplying gear. Since the 1930's this
   has been the most common type of bicycle, now simply called
   bicycle. The older high-wheelers are often referred to as
   bone-rattlers.
   [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

safety bicycle
    n 1: bicycle that has two wheels of equal size; pedals are
         connected to the rear wheel by a multiplying gear [syn:
         safety bicycle, safety bike]